I have been windsurfing for a very long time but since the beginning when trying to get planing I first put my "front foot" in the strap and after that (when the board feels ready to plan) the "rear foot" in the strap. Works fine for me.
During my last trip to the Lake of Garda some guys told me they just do it the other way (rear foot first in the strap and after that the front foot).
I was really confused and argued that was totally wrong and would make it much more difficult to get the board planing - at least in low wind conditions. I failed to convince them, but maybe I was wrong?